The present invention generally relates to a method and apparatus for driving a solid scan type recording head, and more particularly to a method and apparatus for driving a solid scan type recording head in which a plurality of elements having a function of having light emission, exothermic or discharge are arrayed.
Currently, there is known a solid scan type recording head such as an optical recording (multi-stylus) head, a thermal head, and an electrostatic recording head. Examples of an optical recording head are a light emitting diode array (an LED array), a liquid shutter array, and a fluorescent dot array. As a thermal head, welding coloring type and thermal image transfer type are known.
Generally, there is the difference in characteristics such as recording power (a dose of exposure) among manufactured solid scan type recording heads. Additionally, there is a difference in characteristics of focusing elements arranged in a focusing element array used for focusing light emitted from each focusing element. For these reasons, unevenness occurs in recording quality in case where each element is driven by the same driving control. From this viewpoint, an improved driving method has been proposed, in which a printing time (drive time) is changed for every element. However, the shape of recorded dot images is uneven due to the difference in printing time, and therefore ununiformity of recording occurs.
In order to eliminate the above-mentioned problem, a further improved method has been proposed in Japanese Laid-Open Patent Application No. 62-241469. In the proposed method, a voltage or current application time for each element (an LED, for example) is defined by a plurality of reference pulses arranged over a fixed time, and voltage or current is applied to each element over the identical fixed time.
This is further described with reference to FIGS. 1, 2A and 2B. In a case where m light emitting diodes LED1 through LEDm are driven, a plurality of reference pulses are suitably arranged over a fixed application time To (a write time amounting to one dot with respect to the same exposure line) with respect to each of the LEDs. Thereby, exposure energy over the application time To is made fixed with respect to each of the LEDs. For example, a small number of reference pulses is given the LED2 which has a large amount of emission power, while a large number of reference pulses is given the LED1 and LEDm, each of which has a small amount of emission power. As a result, it is possible to obtain the even dot shape depending on the application time To. The above-mentioned proposal can reduce unevenness of the shape of printed dots over the entire line to some extent.
Referring to FIG.2A, Eo is an amount of energy obtained when exposing a light emitting element having an ideal emission power level Po over a time To, that is, ti Eo=Po.times.To. FIG.2B relates to the i-th element having an emission power level Pi (Pi&gt;Po). The i-th element is exposed in such a manner that N reference pulses each having a pulse duration time ti are intermittently applied to the i-th element. An amount of exposure energy Ei obtained at this time corresponds to a value obtained by integrating hatched areas shown in FIG.2B, that is, Ei=Pi.times.ti.times.N.
A number of reference pulses N to be arranged over the fixed time To is calculated by the following formula so as to select exposure energy Ei so as to be identical to ideal energy Eo and thereby eliminate the difference in exposure energy Ei between adjacent dots: EQU N=Eo/(Pi.times.ti)=(Po.times.To)/(Pi.times.ti)
However, even with the proposed method, there is a possibility that the unevenness in density among the elements may occur. As is illustrated in FIG.2B, a portion having emission power Pi and a portion having emission power Po are alternately arranged over the fixed time To corresponding to one dot. The repetition depends on the emission power Pi of an element of concern, and there exists a small exposure energy distribution over time To at a subliminal level. Therefore, ununiformity in density distribution in one dot occurs. Those examples are the distribution of a latent image potential on a photosensitive medium, distribution of adhesive toner quantity after developing, distribution of density of image on an image transferred paper obtained after transferring and fixing images. The above-mentioned ununiformity of density in one dot causes unevenness in printed images and deteriorates recording quality, particularly in high-quality recording and graphics mode.